The present invention relates to a DC-DC converter, and more particularly, to a circuit and method for controlling a step-up/step-down DC-DC converter.
Portable electronic devices, such as notebook computers and game machines, incorporate a plurality of semiconductor integrated circuit devices. Each semiconductor integrated circuit device included in an electronic device is supplied with operating power from a battery. The output voltage of a battery decreases as the battery is discharged. Electronic devices use a DC-DC converter to keep the operating power voltage constant. Three types of DC-DC converters are known, namely, a step-down DC-DC converter, a step-up DC-DC converter, and a step-up/step-down DC-DC converter. The type of DC-DC converter used by an electronic device is determined in accordance with conditions such as consumption power, operation time during use of the battery, size of the electronic device, and weight of the electronic device.
FIG. 1 is a schematic block circuit diagram of a step-up/step-down DC-DC converter 10 according to a first prior art example.
The DC-DC converter 10 performs DC-DC conversion on an input voltage Vi to generate an output voltage Vo.
The DC-DC converter 10 includes a control circuit 11, a choke coil L1, output transistors T1, T2, T3, and T4, and a smoothing capacitor C1. Each of the transistors T1 to T4 is an N-channel metal oxide semiconductor (MOS) transistor.
The first transistor T1 has a drain supplied with the input voltage Vi and a source connected to a first terminal (input side terminal) of the choke coil L1. The second transistor T2, which is used for synchronous rectification and which corresponds to the first transistor T1, has a drain connected to the first terminal (input side terminal) of the choke coil L1 and has a source connected to a low-potential power supply (ground).
The third transistor T3 has a drain connected to a second terminal (output side terminal) of the choke coil L1, and has a source connected to the low-potential power supply (ground). The fourth transistor T4, which is used for synchronous rectification and which corresponds to the third transistor T3, has a drain connected to the smoothing capacitor C1 and a source connected to the second terminal (output side terminal) of the choke coil L1.
The gates of the first transistor T1 and the third transistor T3 at the main switching side are provided with a first control signal DH from the control circuit 11. The gates of the second transistor T2 and the fourth transistor T4 at the synchronous side are provided with a second control signal DL from the control circuit 11. The first transistor T1 and the third transistor T3 are turned on and off at the same time. The second transistor T2 and the fourth transistor T4 are turned on and off at the same time.
When the first transistor T1 and the third transistor T3 are turned on and the second transistor T2 and the fourth transistor T4 are turned off, current flows through the choke coil L1 so that energy is accumulated in the choke coil L1. Then, when the first transistor T1 and the third transistor T3 are turned off and the second transistor T2 and the fourth transistor T4 are turned on, the energy accumulated in the choke coil L1 is discharged via the second transistor T2. The on-time of the first transistor T1 is the time during which current flows through the choke coil L1. Energy is accumulated in the choke coil L1 in accordance with the on-time of the first transistor T1. The value of the output voltage Vo is determined by the energy accumulated in the choke coil L1, which is in accordance with the on-time of the first transistor T1 and the second transistor T2. The smoothing capacitor C1 smoothes the output voltage Vo.
A feedback signal FB having the output voltage Vo is fed back to the control circuit 11. An error amplifier 12 included in the control circuit 11 amplifies the voltage difference between a divided voltage, which is obtained by dividing the feedback signal FB using resistors R1 and R2, and the voltage of a reference power supply e1 to generate an amplified signal. A pulse width modulation (PWM) comparator 13 compares the amplified signal of the error amplifier 12 and a triangular wave signal of a triangular wave oscillator 14 to generate the control signal DH, which has a pulse width that is in accordance with the comparison result, and a control signal DL, which is complementary to the signal DH.
When the energy accumulated in the choke coil L1 decreases, the output voltage Vo decreases, and the divided voltage obtained using the resistors R1 and R2 becomes lower than the voltage of the reference power supply e1, the first transistor T1 and the third transistor T3 are turned on. Then, when the output voltage Vo increases, the output voltage of the error amplifier 12 decreases, the on-time of the first transistor T1 and the third transistor T3 is shortened, and the on-time of the second transistor T2 and the fourth transistor T4 is lengthened. When the output voltage Vo decreases, the output voltage of the error amplifier 12 increases, the on-time of the first transistor T1 and the third transistor T3 is lengthened, and the on-time of the second transistor T2 and the fourth transistor T4 is shortened. Such operation keeps the output voltage Vo maintained as a constant voltage based on the reference power supply e1.
In the DC-DC converter 10, the first transistor T1 and the third transistor T3 are turned on and off at the same time, and the second transistor T2 and the fourth transistor T4 are turned on and off at the same time. In other words, a large number of transistors operate at the same time. This results in operation loss and lowers efficiency of the DC-DC converter 10.
FIG. 2 is a schematic block circuit diagram of a DC-DC converter 20 according to a second prior art example. A control circuit 21 of the DC-DC converter 20 controls first and second step-down transistors T1 and T2 separately from third and fourth step-up transistors T3 and T4. More specifically, the control circuit 21 includes a first PWM comparator 22 and a second PWM comparator 23. The first PWM comparator 22 generates control signals for controlling the step-down first and second transistors T1 and T2. The second PWM comparator 23 generates control signals for controlling the step-up third and fourth transistors T3 and T4. A voltage supply e2 is connected between an inversion input terminal of the second PWM comparator 23 and a triangular wave oscillator 14. A triangular wave signal of the triangular wave oscillator 14 is offset by an amount corresponding to a DC voltage of the voltage supply e2. The second PWM comparator 23 is provided with the offset triangular wave signal.
The first PWM comparator 22 compares the triangular wave signal of the triangular wave oscillator 14 and an amplified signal of the error amplifier 12 to generate the control signals DH1 and DL1 based on the comparison result. The second PWM comparator 23 compares the triangular wave signal of the triangular wave oscillator 14 to which the voltage of the voltage supply e2 is added and the amplified signal of the error amplifier 12 to generate the control signals DH2 and DL2 based on the comparison result. Thus, the control circuit 21 operates the step-up transistors T3 and T4 or the step-down transistors T1 and T2 in accordance with the level of the output voltage Vo. In this way, the DC-DC converter 20 operates as a step-up DC-DC converter or a step-down DC-DC converter depending on the level of the output voltage Vo. Although the DC-DC converter 20 includes the four transistors T1 to T4, operation loss that would be caused when activating and inactivating two transistors at a time is avoided.